How To Prove You're A Time Traveller

How do you convince people you aren't a witch if you travel back in time, and where can you actually find something decent to eat when you're there? Professor Hannah Fry and VSauce, Michael Stevens, tackle the most practical and delicious logistics of surviving a trip through history. Check out one of Hannah's favourite chemistry YouTube channels - https://www.youtube.com/@cnliziqi ------------------- For more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit ⁠⁠https://cancerresearchuk.org/restisscience⁠⁠ Cancer Research UK is a registered charity in England and Wales (1089464), Scotland (SC041666), the Isle of Man (1103) and Jersey (247). A company limited by guarantee. Registered company in England and Wales (4325234) and the Isle of Man (5713F). Registered address: 2 Redman Place, London, E20 1JQ. ------------------- Find The Rest Is Science all over the internet by ⁠⁠clicking here.⁠⁠ ------------------- Video Producer: Adam Thornton + Oli Oakley + Jack Meek Animator: Sam Benson Video & Social: Bex Tyrrell Assistant Producer: Lucy Lipscombe Producer: Simona Rata Senior Producer: Lauren Armstrong-Carter Head Of Digital: Samuel Oakley Exec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Hannah Fry?

  Welcome to The Rest of Science. I'm Hannah Fry.

• 0:03Michael Stevens?

  And I'm Michael Stevens.

• 0:04Hannah Fry?

  And this is Field Notes, which is the part of the week where Michael and I are supposed to take a bit easier to turn up and just answer your questions and show each other an object and have a lovely old time. And frankly, you guys don't notice the amount of effort that we put in in the other one because you like this one just as much.

• 0:24Michael Stevens?

  Well, you know what? It's the magic that we have, Hannah. We should try to do something really boring. Like, how few views can we get on a podcast episode?

• 0:35Hannah Fry?

  Make that the challenge, yeah. See what we can optimize for.

• 0:39Michael Stevens?

  We could do just like a clip show, like, not a best of clip show, but a worst of clip show. The moments where jokes fell flat, where we said something kind of wrong.

• 0:52Hannah Fry?

  Unfortunately, it's not going to be today because it's absolutely, it's jam packed full of amazing stuff.

• 0:56Michael Stevens?

  I know. Today's is actually good. Today, I want to show off a tool I invented that helps solve a problem that will never happen.

• 1:04Hannah Fry?

  We've done something a bit different today. We've done something a bit upside down. We've gone through the mailbag and we have found questions that are all to do with time travel or sort of unexpectedly popping out of existence in one place and popping up somewhere else.

• 1:19Michael Stevens?

  So I'm going to begin with a question for you, Hannah, and this is also for all the listeners out there.

• 1:23Hannah Fry?

  Okay.

• 1:24Michael Stevens?

  If I were to time travel you right now, at this instant, with what you have on your person, to today, okay, this exact same day, but in 2007.

• 1:37Hannah Fry?

  Right.

• 1:38Michael Stevens?

  How would you prove that you were from the future?

• 1:48Hannah Fry?

  This episode is brought to you by Cancer Research UK.

• 1:51Michael Stevens?

  Here's something strange. Your DNA contains more ancient viral fragments than genes. The genes that build our cells make up only 2% of our DNA. And for years, that is what scientists focused on. They treated the rest, the ancient. Viruses and stuff as junk.

• 2:12Hannah Fry?

  But now we know that that hidden majority, sometimes called the dark genome, influences how our biology works and how diseases like cancer behave.

• 2:22Michael Stevens?

  It's a reminder that progress rarely comes as a single breakthrough. It builds gradually. Cancer Research UK plays a central role in that progress, supporting decades of research into over 200 types of cancer, work that's helped double survival in the UK over the past 50 years.

• 2:41Hannah Fry?

  For more information about Cancer Research UK, their research breakthroughs, and how you can support them, visit cancerresearchuk.org/the rest is science.

• 2:54Speaker 02

  This episode is brought to you by Indeed. You're ready to level up your business, but the pressure to find the right person to make that happen is intense. Indeed sponsored jobs can help carry the load. It boosts your job post to help it reach quality talent fast. Cut through the KOs with Indeed sponsored jobs. Get a $75 sponsored job credit at indeed.com/podcast. Terms and conditions apply. Hiring now? Then this is a job for Indeed sponsored jobs.

• 3:24Speaker 03

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• 4:01Hannah Fry?

  2007.

• 4:03Michael Stevens?

  Yeah, so you're in the same room that you're in right now. If you're listening and you're in the car or you're at home, boom, it's just suddenly 2007. What do you do? How do you get anyone to believe that you actually came from 2026? This whole challenge of a time traveler being able to prove that they're from some when else in time? I call it the time traveler's credibility problem.

• 4:27Hannah Fry?

  Nice.

• 4:28Michael Stevens?

  There's just too many easier explanations for their behavior and knowledge than the truth, which is that they actually traveled through time. How do we crack it?

• 4:37Hannah Fry?

  Do I get advance notice that I'm gonna go like, can I prep? Can I bring stuff with me?

• 4:40Michael Stevens?

  No, no advance notice.

• 4:42Hannah Fry?

  Just straight away there?

• 4:43Michael Stevens?

  Instantly. Or you could imagine that you accidentally time traveled. You had no preparation. You can't bring anything with you except what you have on your person.

• 4:51Hannah Fry?

  Okay, so I'm trying to think 2007, because the thing is, is that there's, you could say, oh, in the year 2020, there's gonna be a big pandemic. But then people have got to wait 13 years for that to happen.

• 5:04Michael Stevens?

  Exactly.

• 5:04Hannah Fry?

  So that's not gonna work. I don't have any lottery results memorized.

• 5:08Michael Stevens?

  Nope.

• 5:10Hannah Fry?

  I don't have like a, you know, a catalog of like number one hits in my head that would be like, oh, I know that this person's gonna be a big deal. I mean, every bet that you could lay would take some time to prove. How quickly do I have to prove that I am, that I am from the future?

• 5:29Michael Stevens?

  Well, as fast as possible, right? That's the goal here. And the more you think about it, the more you realize how difficult it is. Because even if I gave you some prep time and you brought an almanac with you or a list of lottery numbers, you run the butterfly effect risk where just by you being there and having this book and speaking the numbers, you cause, you know, a little bit of a wind current change, which changes how the balls are collected. And the number doesn't even match what it should have been if you hadn't been there. Therefore. Nullifying your attempt to prove you're from the future.

• 6:05Hannah Fry?

  Like Biff's dad in, um, Biff and his dad in Back to the Future, where they swap roles.

• 6:10Michael Stevens?

  Exactly. Uh, and similarly, if someone were able to predict the Lotto numbers, like, I don't know, three days in a row, I still wouldn't think they were from the future. I would think that they were in cahoots with the lottery. That's a much simpler explanation. So what do you do?

• 6:26Hannah Fry?

  What do you do, Michael?

• 6:27Michael Stevens?

  Well, I'm not entirely sure, but I think I've gotten as close as I can with my own brain. I created last year some slap bracelets that look like this. And I've got them on wooden hands because, you know, you gotta, you gotta have them modeled.

• 6:43Hannah Fry?

  They were pretty fancy in seven, in 2007, to be fair.

• 6:47Michael Stevens?

  Yeah, exactly. Right? Um, and this is good for any time travel from 20, let's see, from 2022 all the way back to the year 600.

• 6:57Hannah Fry?

  Okay.

• 6:57Michael Stevens?

  So if you're gonna go before 600, you'll need a different slap bracelet. If you're gonna go back just like a year, you'll need a different slap bracelet. But this thing will stay on you at all times. And it is an emergency tool to prove that you've traveled through time to, to the people in the past. The way it works, and I want to hear what your thoughts are, because I, I'm not saying this is perfect, but what this gives you is a timeline with some bars on it, showing famous, widely known things that were lost and then later found.

• 7:31Hannah Fry?

  Oh.

• 7:33Michael Stevens?

  So.

• 7:34Hannah Fry?

  So hold on. We know where they are now because we live in the future, but if you go back in time, people don't know where they're hidden.

• 7:43Michael Stevens?

  Exactly. Because you can't just go back with like a diagram of an iPhone to, say, the 50s and show people, see, this is a future technology. They'll just think that you're really smart and came up with it. You know?

• 7:56Hannah Fry?

  But could you go back with the exact coordinates of the Titanic?

• 7:59Michael Stevens?

  Yes, you could. And so this slap bracelet gives you. The timeline and the GPS coordinates. So if you go back to say, let's say you pull a Marty McFly, you go back to the 1950s. You can say, okay, guys, well, the Titanic has been lost, but it hasn't been found yet. But it can be found at 41 degrees north, 49 west and some change. And they'll go, wow, how did you know that? You must, you must be a sea expert. And you're like, no, no, no, I can also tell you where, uh, let's see, what's another good one for the, for the eighties? I can also tell you where the HMS Endurance or a Shackleton ship is. Oh, nice. So they're stuck going either this guy independently discovered these two sunken ships

• 8:43Michael Stevens?

  or traveled through time, in which case I believe time travel is the better explanation. We've also got Archimedes' asthmachion, the, the method of mechanical theorems, which was lost in 1204 and not recovered until 1906. We've got the Baltimore Gold Horde, which was lost in 1856 and found in 1934. The Sutton Who Ship Burial from the seventh century, lost until 1938. I don't know what that one is. What's that one? Uh, I can't tell you any more details, Hannah. I'm just helping time travelers. I'm not a history teacher. It was our science writer, Scott Frank. I'll give him a shout out. He's the one who found this. I'm like, they need to overlap. There must always be at least two things for every year.

• 9:26Michael Stevens?

  Because you got, you can't just prove it with one. You could just have got lucky. Can't prove it with one. With one, you're just a good, you're just a good looker. But with multiples, especially different types of things, like a shipwreck, a gold horde and an ancient manuscript, come on, you're a time traveler. You're from a time when these things have been found. And unlike the, the result of a sporting event or a lottery, you can immediately have people go out and verify your claim. Hang on, I want to know whether you worry on your person at all times. No, I don't. Um, I'll admit that. This feels like a flaw in your plan, Michael. I know. It's, you know, I don't anticipate accidentally time traveling at any point. But man, wouldn't it be hilarious if that happened?

• 10:10Michael Stevens?

  And I was like unprepared and I'm like, man, I specifically made a tool for just this. And I don't have it.

• 10:18Hannah Fry?

  Yeah, that would be the worst.

• 10:19Michael Stevens?

  The dream is to eventually make an entire like pocketbook that has much more than just a few items on it and that it will allow you to travel through all of of human history, you know? And that's going to have to teach you how to speak the languages you might encounter.

• 10:40Michael Stevens?

  If you, you know, you need to be able to speak Middle English perhaps, or who knows what, right? We'll try to put that together. We also made a slap bracelet that's a ruler. It's got inches and centimeters on it. But since a slap bracelet

• 10:55Michael Stevens?

  wraps around.

• 10:57Hannah Fry?

  You've never looked more cool, Michael.

• 10:59Michael Stevens?

  I've never looked more cool. Imagine it looks it looks more like a medical bracelet, like, oh, this guy's got some kind of.

• 11:04Hannah Fry?

  It does a bit. It just escaped from the hospital.

• 11:07Michael Stevens?

  I just came from the hospital. But we also made one that takes advantage of this wrapping effect. And it is not a ruler. It's a pie tape. So instead of having a mark every centimeter and every inch, we put a mark every pie centimeters and every pie inches. So from here to here, that gap between my two thumbs is four pi centimeters. Why would you want to know that? Well, if I wrap that around something, what I'm finding, when I look at four pi centimeters, that's the circumference, which means the diameter is uh, four centimeters.

• 11:47Hannah Fry?

  So you can instantly measure the diameter of something.

• 11:50Michael Stevens?

  You can instantly measure the diameter of something. Like, imagine you you find a pipe and the ends of the pipe are inaccessible. So you can't like actually just go across the diameter of the pipe to tell. You have to go around it and divide by pi, not with the pie tape. You just go around. The number it reads is the diameter.

• 12:08Hannah Fry?

  This is extremely useful. I I do feel like making these tools and then not wearing them is your real downfall here, though. I think I think you need to commit. I think you need to start tattooing some of these onto your body.

• 12:18Michael Stevens?

  I could tattoo them on my body, yeah. The one fear, though, is that the the existence of the tattoo itself might make me look like a witch. I mean, the knowledge of where these things are might make me look like a witch. If I went back to like the 1100s and I'm like, hey, guys, I know where this Archimedes thing is and blah, blah, blah, they might say, mm, okay, either you're from the future or you're in a pact with the devil. I haven't actually solved that problem, have I?

• 12:48Hannah Fry?

  I do like the idea of you going to the, uh, the center in Paris where they have the, the definition of the meter and you've been like, hang on a second and just whopping out your leg and being like, look, this is, uh, I've got the measurement on me. You know, my sister, who makes a lot of her own clothes, she's been discussing for a while now, having an inch tattooed onto her thumb.

• 13:08Michael Stevens?

  Ah, yes.

• 13:09Hannah Fry?

  Just so that you can do an inch hem. Isn't that, I like that idea.

• 13:12Michael Stevens?

  Yeah, it's a really good idea. Uh, so Adam Savage is someone I know in real life who has a ruler tattooed on his body.

• 13:18Hannah Fry?

  Does he? Is it accurate?

• 13:20Michael Stevens?

  Well, no, it's symbolic, you know? Um, the problem is that your, your body changes, your skin changes, and so the inch changes. Um, but again, as a, as a sign of like, I'm a maker and I'm, I'm a measurer and I love quantities and quantification, it's a great tattoo to have. But we struggled with this, um, at the Curiosity Box, trying to come up with a shirt that had rulers on it because inevitably the shirt will stretch, the shirt is, is a, is a material that moves, it's just not useful as a measuring tape. It can be useful for, you know, formulas and conversion tables, that's great. We've done two shirts that have that property.

• 14:05Michael Stevens?

  But, uh, this is so far the best I've been able to come up with. And I'm just now realizing that this is the prototype. The real ones we made were adult sized. Look at that.

• 14:15Hannah Fry?

  Ah, more space for more coordinates.

• 14:18Michael Stevens?

  This is for your time traveling children. This is an adult slap bracelet. It is like really long, by the way.

• 14:25Hannah Fry?

  You're gonna wear it as a neckerchief.

• 14:27Michael Stevens?

  You could, yeah, uh, let's see if I can.

• 14:32Michael Stevens?

  Oh, no, I can't. Look at that.

• 14:34Hannah Fry?

  Any of you who are listening rather than watching, you just missed the absolute delight of watching Michael Stevens of Feasible Spain slap himself in the neck with a slap bracelet.

• 14:47Michael Stevens?

  I'm kind of a Gastoni guy. My neck is too thick. Even my pie tape won't tell me the diameter of my neck. It's just not long enough. Well, that'll be for a future item. Look at that. See? Doesn't make it.

• 15:00Hannah Fry?

  So close.

• 15:01Michael Stevens?

  So close.

• 15:02Hannah Fry?

  Careful now. You have to lift up your head quite far to get your beard out of the way. I don't want you to get your top job there to go.

• 15:07Michael Stevens?

  I know. Isn't that funny? Like, my beard is so long now. It's changed. It's been long for so long. It's changed even my own perception of what my head looks like. I think of my head as being this huge thing. But actually, my head is like a little bean. Look at this guy.

• 15:26Hannah Fry?

  Yeah.

• 15:27Michael Stevens?

  My daughter looked at our wedding photos the other day and she was like, "Dad used to be so small." And it's because my head used to be half the length.

• 15:36Hannah Fry?

  I'm slightly disappointed, I've got to be honest with you, that when you pulled your beard back there, your head was small underneath. I really, I really, I really wanted you to have a jaw all the way down.

• 15:46Michael Stevens?

  I don't. I don't have anything else to share about my object. I'm open to all kinds of questions and comments. I did just finish a time travel book. Bid Time Return by Richard Matheson. I love it because the method of time travel is literally just believing strong enough that you've traveled.

• 16:05Hannah Fry?

  I mean, look, some forms of time travel are not necessarily completely impossible, right? It's just the going backwards that's going to be difficult.

• 16:12Michael Stevens?

  Exactly. I can travel through time forward at regular speed effortlessly. That's just called living. Sleep is pretty good, too. With sleep, you can travel through time with your consciousness. Like, the travel seemed to not happen.

• 16:31Hannah Fry?

  If you did jump forwards in time, though, rather than backwards, how could you prove you were from the past?

• 16:37Michael Stevens?

  Whoa.

• 16:39Hannah Fry?

  That's really tough, isn't it?

• 16:40Michael Stevens?

  I don't know.

• 16:42Hannah Fry?

  Where's the slap bracelet for that?

• 16:43Michael Stevens?

  I know, because what would you do? Because your only evidence would be your own ignorance of things. And that, anyone can have that.

• 16:50Hannah Fry?

  Unless there's something the other way around. Unless there's something that we have now that is lost to the future.

• 16:55Michael Stevens?

  Okay. In that case, you need to carry around with you a very, very priceless relic that's well known. So that when you jump forward, people will say, oh, hey, that, you know, medieval chalice has been missing since you jumped. And you're like, I know I've got it because it came with.

• 17:13Hannah Fry?

  Here it is. Or what about a skill? You know, you could say, um, uh, ask me a phone number. I know six different phone numbers, you know? And people in the future just won't, won't know any phone numbers. Or like, I can write with a fountain pen.

• 17:31Hannah Fry?

  I still, because actually I think if you got somebody from, you know, whatever, the 1700s, they would probably have skills that modern people know. I'm sure they could weave a basket, you know?

• 17:41Michael Stevens?

  Right, right.

• 17:43Hannah Fry?

  Make ice cream in the traditional way.

• 17:46Michael Stevens?

  Yes. So skills, obviously, although they're funny, the skills anyone could learn, you could learn to read cursive, even though it's the year 2700. Uh, I don't know why I'm anticipating the demise of cursive. I've just always felt that it was confusing.

• 18:05Hannah Fry?

  And difficult to read. You're right. It's going to be difficult to prove.

• 18:08Michael Stevens?

  I, I have a friend who I was talking to about this time travel idea. And I wanted to do it as a Vsauce episode. It became, it became a product, but she said it should just be a book. It should be a science fiction novel about time travelers and how they have to, you know, have slap bracelets like this or almanac books they carry. We didn't think about going to the future. In which case, yes, there needs to be like a big collection of important relics that are always kept track of. And you get to carry one with you when you travel through time to prove that you're, you, you are the time traveler and that you're from the past.

• 18:44Hannah Fry?

  Maybe we're overthinking this. May, maybe it's just that, um, all you need is to prove that you know how slap bracelet works. You know? Maybe that's the skill that's going to be lost to the future. And maybe you get a two in one.

• 18:56Michael Stevens?

  Yeah, you could say, hey, guys, I remember the joy of a plastic straw.

• 19:02Michael Stevens?

  What's that tweet? It's like, look, I just want a straw that's somewhere in between, dissolves immediately and kills turtles and destroys the rainforest. Just somewhere in between would be nice.

• 19:13Hannah Fry?

  Is there nothing in between? Bamboo, you know, maybe, maybe bamboo. Should we have a quick break first? Let's do that.

• 19:28Speaker 02

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• 19:57Speaker 03

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• 20:35Michael Stevens?

  All right, and welcome back. We are having the time of our lives answering and reading your questions about time. Here's one for Hannah. It comes from Alex. Watching a documentary about the dinosaurs got me thinking. If somehow a human ended up on Earth 150 million years ago.

• 20:54Michael Stevens?

  Would they be able to find food they can digest? What is the earliest time when we would be able to find food?

• 21:01Hannah Fry?

  This is such a good question. This has never occurred to me. I like how your mind works, Alex, because until you mentioned this, I just honestly hadn't even thought about it. But you are totally right, because 150 million years ago, this is the late Jurassic period, there's no flowering plants. I mean, you can go back and listen to our episode on what's the most vegetable vegetable to discover that you're just.

• 21:22Michael Stevens?

  Wait, wait, wait, wait, I didn't know this. So there was no fruit?

• 21:25Hannah Fry?

  No fruit, no grasses, there's no legumes, there's no root vegetables.

• 21:31Michael Stevens?

  Surely I could eat a dinosaur.

• 21:34Hannah Fry?

  I mean, sure, meat.

• 21:36Michael Stevens?

  Meat.

• 21:37Hannah Fry?

  Well, you can accept that. Well, okay, two things. First off, it would be like chewing on an old goat, right? Like, the meat would be so tough, it would be really horrible.

• 21:48Michael Stevens?

  One of my favorite foods, go on.

• 21:51Hannah Fry?

  But also loads of the stuff, loads of the, I mean, you're basically in fern territory. There's like conifers, there's loads, it's like big green stuff, you know? Like, you've got conifers, you've got ginkgos, and you've got cycads, I think they're called.

• 22:09Hannah Fry?

  And the problem is, is that all of the dinosaurs are eating those, fine, but they are totally loaded with carcinogens. So it's almost certain that the dinosaurs themselves would be carcinogenic.

• 22:23Michael Stevens?

  No kidding?

• 22:25Hannah Fry?

  So, I mean, your options are basically eat nothing or cancer. So enjoy. Basically, none of the aisles of the supermarket exists. You've got, I mean, you've sort of got fiddleheads, which are like the coiled fronds of young ferns. And you've got something that's like kind of old school pine nuts, like prehistoric pine nuts, like conifer seeds. But the thing is, is that, you know, even if you're trying to eat the vegetation and it's not carcinogenic, even if you manage to find some stuff like that, modern humans, we've basically lost the ability to ferment really high fiber, kind of woody plant material efficiently.

• 23:08Michael Stevens?

  Yeah.

• 23:08Hannah Fry?

  And Jurassic plants, they are way tougher, way more fibrous than the soft vegetables that we get today. So it would be a lot less like eating salad and more like eating furniture, essentially. It's gonna be, it's not gonna be very fun. And then also loads of the foods that we eat now, we've like bred the toxins out of them. You know, things like almonds, potatoes, we've like bred them over the years so they don't kill your your kidneys or your liver. So so I mean, you could survive, but you've basically got roasted dinosaur and pine nuts and that's it. That's all you got. You got you can't even flavor them. You know, chilies don't even exist for another hundred million years. It's um it's gonna be boring.

• 23:46Michael Stevens?

  Wow, I had never thought of that before, but yeah, I mean, a hundred and fifty million years ago, you can go further than that before they're even early mammals. And then you're really in trouble because the mammalian diet needed to be there for mammals, which we are, to exist. Now, I feel like I could maybe eat some fish, but you're saying that the food chain is probably pretty contaminated with stuff that's that we're not ready for.

• 24:13Hannah Fry?

  Absolutely, absolutely. I mean, you probably would be able to, you probably would be able to, but it's not, I mean, you're really gonna struggle. Also, by the way, it's not just the food, because if you're going back a hundred and fifty million years ago, the CO2 levels are five to ten times higher than you've got today. So you are gonna be perpetually out of breath, basically. You might be trying to hunt down a dinosaur to eat them, but you're gonna you're gonna be suffering from CO2 toxicity, just just trying to get dino steak.

• 24:43Michael Stevens?

  That's wild. How have I never thought of that before? Yeah, of course. Like, it'd be easier to go to Mars than to go live in the like early Jurassic, but you're on Earth, and yet it's just not for you yet. The air isn't right. If you're a vegan, sorry. If you're willing to to hunt living creatures, you don't have any chicken. Um, you've basically got fish, sharks, reptiles.

• 25:14Michael Stevens?

  Other than that, you've got a bunch of like tree bark.

• 25:17Hannah Fry?

  Yep, bit of tree bark. That's it. Not even any grass. Can't even chew on any grass.

• 25:22Michael Stevens?

  So no wheat?

• 25:23Hannah Fry?

  No wheat, no grains, nothing. Yeah, it's gonna be, it's gonna be, you're gonna have a terrible time. And no one around to tell about the bracelet. Um, okay, here's a question from Adam. I really like this question because of course, everything that's going on with Artemis at the moment. That amazing expedition around the dark side of the moon. Adam asks, what would the lag on a FaceTime call be like from space or from the moon?

• 25:51Michael Stevens?

  Oh, yeah. What would it be like? So, you know, the, the lag that we get in FaceTime calls is caused by all kinds of factors, but ultimately, at the very end, even if your technology is perfect, you're always bound by the speed of light. The speed at which an electromagnetic signal travels. It's, it's, it's finite. And it's really fast, but when you talk about really long distances, it becomes a big problem. So for the astronauts on Artemis 2, at their, at their furthest point, well, I mean, at their furthest point, they couldn't contact Earth at all, right? For 40 minutes, they were out of contact with Earth because the moon was in the way.

• 26:34Michael Stevens?

  They couldn't radio. And just as a side note, on the Apollo missions, when people actually left and, and walked around on the moon's surface and someone stayed up in the command module, that one person in the module was alone on the far side of the moon by themselves, with no radio contact with anybody. And the first guy to do that was Michael Collins, who also, I think, went the furthest away, but he did it alone. Artemis 2 crew, they have each other. Michael Collins was alone and he was the furthest any human has ever been from any other human. Even Buzz and, and Neil on the moon's surface were further away from him than you can be

• 27:17Michael Stevens?

  from another human here on Earth, on the surface, yeah.

• 27:20Hannah Fry?

  Because he was orbiting around the dark side.

• 27:22Michael Stevens?

  Well, the far side, let's call it, because the dark side is always changing. But yeah, he was, he was on the far side of the moon by himself in the command module. And the closest people were the two other guys on the surface of the moon on the other side. I don't, no humans have ever been on, on, on the moon at night. By the way.

• 27:41Hannah Fry?

  Is that true?

• 27:42Michael Stevens?

  Yeah.

• 27:42Hannah Fry?

  Wow.

• 27:43Michael Stevens?

  Yeah. So, so, I mean, it would kind of make sense. Every, every crude mission to the moon has happened during the lunar day. And it makes sense because at night, it would be harder to see things, right?

• 27:56Hannah Fry?

  You want to be able to see where you're going.

• 27:57Michael Stevens?

  But what that means is that no human has ever experienced earth shine alone. At night on the moon, it's actually quite bright because the earth is, you know, when there's a full earth on the moon, it's like a full moon, but times like 20, because the earth is bigger and it's brighter. The moon is actually pretty dusty gray. But, but earth is very white because of the clouds and the ice caps. So night on the moon is just not nearly as dark as what we get here on earth.

• 28:30Hannah Fry?

  Oh, that's so beautiful.

• 28:31Michael Stevens?

  Yeah, right? I mean, night on the moon with a, with a full earth is going to give you like pretty distinct shadows at night on the moon because of earth. But, look, I'm actually going to answer your question. The, the, the light speed delay from earth to Artemis 2 is, at its maximum, was 1.357 seconds. That's how long it took a light speed message to get from mission control on earth to the Artemis spacecraft. And that's just one way.

• 29:02Hannah Fry?

  Just because of how far away it is. Incredible.

• 29:04Michael Stevens?

  That's how far away it is. And so, like, just to give you guys a sense of the radio delay, because of how far Artemis 2 is, Hannah and I are going to have a conversation with that 1.357 second delay, thanks to our editors. Okay. Hannah, how are you?

• 29:25Hannah Fry?

  I'm good, Michael. How are you? How's the earth shine?

• 29:30Michael Stevens?

  The earth shine is fantastic, but I'm not loving this 1.357 second delay.

• 29:36Hannah Fry?

  It does slightly get in the way of really having a good conversation.

• 29:44Michael Stevens?

  And so there you go. That is the delay between the Earth and the moon.

• 29:50Hannah Fry?

  Hey, Mom, I'm on the other side of the moon.

• 29:53Michael Stevens?

  Yeah, did we fool you? Thanks editors for recreating, for demonstrating that fact. But when you get further than the moon, things get really messy. I mean, Mars, okay, we're gonna, yeah, let's go to Mars, all right? Mars is so much further away. And it's also much more variable. The moon is sometimes a little bit closer, sometimes a little bit further away. But I think that the difference in its distance is, literally, creates a light speed delay difference of only about a tenth of a second between when it's at its closest and furthest from Earth. But for Mars, that radio message delay is going to be anywhere from three minutes

• 30:36Michael Stevens?

  at its closest to 21 minutes.

• 30:41Michael Stevens?

  Three minutes. We're not talking about seconds anymore.

• 30:44Hannah Fry?

  That's not a conversation anymore, is it? That's, that's, that's voice notes. You're, you're in voice note territory.

• 30:48Michael Stevens?

  That's right. It's voice note territory. It's like, okay, even when they're, I mean, three minutes is actually pretty long to wait to be like, hey, hello, are you okay? And then you wait three minutes just for them to get the message. Then they have to reply. And then, once they send, you have to wait three more minutes. So it's at least six minutes minimum.

• 31:09Hannah Fry?

  Or up to 40.

• 31:11Michael Stevens?

  Up to 40. And once you get to Jupiter, that's 33 to 53 minutes, one way.

• 31:21Michael Stevens?

  Pluto, five hours.

• 31:24Hannah Fry?

  Is it? Goodness me.

• 31:26Michael Stevens?

  Five hours.

• 31:29Hannah Fry?

  Do you know what, though? That also really demonstrates how much closer we are to the Sun. I mean, the Sun is nine minutes light speed, right, from us?

• 31:35Michael Stevens?

  Yeah.

• 31:36Hannah Fry?

  So, and then if we are five hours to Pluto, I mean, that's a really good way to think of the scale of the solar system and just how close we are to the Sun in comparison to the other planets. Wait, how long was Jupiter? An hour, basically?

• 31:51Michael Stevens?

  Jupiter, yeah, between half an hour to an hour, depending on where each planet is in its orbit, right? And then the nearest star that isn't our sun, that gets you out of the hours into the years, right? Four years to Proxima Centauri, the nearest star to Earth. And what's so interesting to me about Proxima Centauri is that it's the nearest star, it's about 4.25 light years away, and yet it's so dim, we can't see it with the naked eye. It's like, guys, I'm right here, I'm the closest to you, and yet we cannot see it unless we have a tool.

• 32:30Hannah Fry?

  Four years really doesn't feel like a very long time. I mean, if you were sort of, if you were giving it an update on what was going on on Earth, and be like, hey, we've got a new president. Oh, no, wait, strike that. We've got a new president. It would be a...

• 32:42Michael Stevens?

  That's a good point, yeah. You would tell them the results of the election, and by the time they got the message, there would have been another election. And then by the time they responded, and you got their response, more than eight and a half years would have passed. And so you would have had, you would have been having the third election already.

• 33:01Hannah Fry?

  By that point, it's just not really worth keeping them in the loop, is it?

• 33:03Michael Stevens?

  No, you wouldn't. And this isn't something that technology is gonna fix. Well, I mean, it could, I guess. But this is the limit of the speed of light. We would have to discover something very brand new about spacetime. For example, create wormholes for instantaneous communication across these light-year distances. Otherwise, the separation is essentially total.

• 33:28Hannah Fry?

  I'll tell you what, actually, talking about wormholes, we had in another question about wormholes, the downsides of wormholes, let's say. This was AndersTX on YouTube, this is one of the comments on YouTube. Great question. If we had wormholes connecting to each other's homes, would there also be a rush of air through the wormhole due to pressure differentials? Like, if you were connected from somewhere at sea level to another location at high elevation, like Denver or Mexico City? And the answer, absolutely, yes, there would. It would be a complete nightmare.

• 34:01Hannah Fry?

  You guys are really good at thinking about the the the real caveats of of instituting this, you know, time travel wormhole, what are the downsides? Okay, so of course, air moves from from high pressure to low pressure. At sea level, air pressure is 14.7 PSI, right? In Denver, which is at 5,000 odd feet, not far from you, Michael, right? It's 12.1 PSI. Mexico City, by the way, it's about 11.1 PSI. So if you open a wormhole in London, right, where I am, to where you are, near Denver, you've got a difference of about, you know, 2.6 PSI, which doesn't sound that small, but in Fluttenhamming's terms, it's going

• 34:44Hannah Fry?

  to be an absolute monster, because you've got this Benui principle, you've got this pressure drop of like 17,000 pascals. Essentially, the air would rush through the wormhole at roughly 150 meters per second. I mean, you're talking this way worse than a category five hurricane, essentially. It was not just like a little rush of air. It would be this absolute deafening, structural failure inducing blast. It would just strip the wallpaper off your walls. It would sort of suck you through through the wall before anything. So yeah, I mean, I would say, careful, careful with what you wish for. It would it would rip the snap bracelet right off your arm, put it that way.

• 35:23Michael Stevens?

  You know, it's it's another way to think of it, too, is that you're pretty near sea level right now, Hannah, more or less.

• 35:30Hannah Fry?

  Yeah, pretty much.

• 35:31Michael Stevens?

  I'm in I'm in Colorado, high enough up that the air pressure where I am is about the same as the air pressure in a Hoover that you would use today.

• 35:45Michael Stevens?

  And by Hoover, I mean vacuum, for those of you who are Americans. And that's that's kind of like blew my mind when I learned that to be like, wow, so this is what it's like to be inside a vacuum cleaner at sea level.

• 35:58Michael Stevens?

  That's literally true. And yeah, that pressure difference is what it takes to hoover up all the debris.

• 36:05Hannah Fry?

  See, the other thing is, though, you'd also have this temperature issue because if you have

• 36:11Michael Stevens?

  Oh, yeah.

• 36:12Hannah Fry?

  You know, the air rushing around, right? It will go under this this expansion because when you get an air that sort of expands rapidly, right, into a much lower pressure environment without losing heat, then its temperature just drops really significantly. So so the air, as it blew out into Denver, because it would go from it would go from London to Denver, right? That would be the direction of the airflow or to Colorado more generally, you know, it would probably drop by tens of degrees instantly. So you would have this this category five, worse than category five, blast of freezing air shooting, shooting into your room.

• 36:51Michael Stevens?

  Yeah. And you would have condensation. So clouds could form.

• 36:55Hannah Fry?

  Mm hmm.

• 36:55Michael Stevens?

  Or or disappear, depending on, yeah, the humidity in the areas, they would have their own weather. Yeah, I hadn't thought of that.

• 37:03Hannah Fry?

  I mean, you would also have me, Michael. I would be there. I probably wouldn't be in one piece. I would probably be frozen and and much mutilated by the journey. But nonetheless, I would be there. So there's upsides and downsides. That's what I'm saying. There's one more question that came in from Jacob Fletcher, and I think this is quite a nice one. This rounds it all off very neatly, actually, going back to your snap bracelet containing the information. At what point did it become impossible for a single person to learn everything that humans had collectively learned, Michael?

• 37:34Michael Stevens?

  I love this question. And of course, it's difficult to actually answer. But the first thing I thought about was a smaller question, which is one that XKCD tackled in What If? And many other people have have taken their own approaches to. And it's the question of at what point could a person have actually truthfully said that they had read every single? Book in English ever written. Because today, that's impossible. And that in and of itself is almost one of the saddest facts. That you and I will never read every book ever written. Even if we read really quickly, and that's all we ever did as a full-time job. If we never slept, we still couldn't catch up.

• 38:18Michael Stevens?

  But if you look at the past, there was a time when there were fewer books that had been written. Now, there's a distinction we need to make between written and printed. Printed becomes much easier to answer because we know a lot more about what was printed on a printing press. And the first book in English that was printed on a printing press was printed in 1473. And it was called The Requiem of the Histories of Troy. That book, printed in 1473, great. Go ahead and read it, and you've read every book ever printed in English. So yeah, you're right. At that point, you read that, you're done. You read that and you're done.

• 39:01Michael Stevens?

  But books get published, and there's this great, there's this great book content creator named Tom Ailing, who I follow on all the different platforms. And he, he went through this like year by year. Because we know about how many books were published every year in English. So he's like, you can actually keep up with this process. I'm not going to get into the hairy details of it all. But Tom Ailing actually did something really interesting. He said, "Don't even start reading with the first English book printed. Just don't even start reading until the year 1500." At which point, there were 294 books in English that had ever been printed, different titles. I could do that. I could do that.

• 39:46Michael Stevens?

  You could do that, and every year, there's like, you know, it looks like maybe 20 new ones printed. And he's like, "You could actually catch up and have read every English book that had ever been printed." Until... 1530. In that year, there weren't 15 or 18 books printed, new books. There were 96. Oh, that's going to be tough. That's one every three days. That's tough. You can't catch up. So 1529 was his estimated year, at which was the last year a person could have ever claimed that they had read every single book ever printed in English. 1529. So unfortunately, we're not there. But that's, that's an answer to a question that's much

• 40:31Michael Stevens?

  more specific. We're talking about reading a book in English that has been printed. Reading in any language, or reading anything in English that's been written, that's, that's where it starts to become basically impossible for this to have ever even existed. To, to have read everything ever written, you would have to talk about some time period just after the invention of writing, because once people were writing, they were writing. Okay? Just, just the accumulation of tax records alone would have meant that, that you would have had to have lived in the Stone Age to have actually read everything. You wouldn't have been able to get access to everything, because there were people writing in China,

• 41:14Michael Stevens?

  in Mesoamerica, in Europe. You'd have to somehow get all this stuff. And it depends on how we define writing. Does, does this also require that you have read all the notches put on bones to represent numbers of, you know, oil jugs shared? But the question that is being asked here by Jacob Fletcher is, when did it become impossible for a single person to learn everything that humans had collectively learned? And that, I think, my answer is never. To, to have learned everything that humans collectively knew? I mean, we don't even know everything we collectively know. And what I mean by that is that we've

• 41:55Michael Stevens?

  got knowledge, but then there's also the meta-memory of it all. Do we know what we know? And we don't always. We've got to refer back to written records. We've kind of externalized our memories. And so I think to know everything that humanity has known collectively, becomes basically...

• 42:18Michael Stevens?

  How do you define collective knowledge? But I'm going to say it was, it's never been possible because of the definitions of these words, which is an unhappy answer. But that's why I also gave you the like, when could you have read every book? Kind of an answer.

• 42:32Hannah Fry?

  Let me think about this, though, because I wonder whether there was a point, I'm just wondering about, I don't know, cats, right? Where cats have different lived experiences. Of course, they do. Each one is like their own individual entity. But if you think about the sort of spectrum of what cats know, right? It's a pretty tightly, tightly packed distribution, right? It's not like, if you take an average cat, right? Sort of imagine the full distribution of everything that all cats know. Take an average cat, there's, it's pretty much going to be overlapping, isn't it really? There's like, there's maybe a couple of little things that some clever cats have learned

• 43:15Hannah Fry?

  here and there, but they're just quite a simple species. So I do wonder whether there was a point where actually the only things that we knew as humans were foraging for food, finding mates, taking care of our young, looking for shelter, you know, where actually there was such simplicity to our knowledge that there just wasn't very much to learn at all.

• 43:37Michael Stevens?

  Yeah, you're right. I'm not really happy with the answer that I gave. I think you're right that if we include sort of instinctive knowledge, then it could be possible to know, oh, it gets cold at night or those kind of things.

• 43:53Hannah Fry?

  Once you get tool use, I think you're right. As soon as you start getting tool use.

• 43:57Michael Stevens?

  Yeah, once you get into, yeah, technology, tools, things that we aren't born already knowing, things that we have discovered and invented, then I guess it's tough because you could say, well, yeah, do you know how to nap a stone to create like a stone blade? Do you know how to make paper? Do you know how to interact with a, with a dog? Like these things that people invented, oh man, I think there really is an answer to this question, at least a hand wavy one where you're like, well, you know. The fermentation of liquids into alcohol happened around this point and that could be a skill that you learned and you could also learn.

• 44:42Michael Stevens?

  But it's, hold on, it's gotta be things that we collectively know. And that's what I'm getting stuck on. Because to know how to use a musical instrument that was invented in China and Egypt and in the Americas, that's not collective knowledge. Only certain groups have that knowledge. So what does it mean, the collective knowledge of humanity? I guess if someone were to write a book of everything that humans like invented and discovered, could you know everything in that book? And at what point was there just too much in it for someone to even be familiar with everything in it?

• 45:17Hannah Fry?

  You know, I also think that actually, human knowledge and technology, I mean, one of my guilty, it's actually not guilty pleasure at all, it's a, what's the opposite of guilty? Proud pleasure.

• 45:28Michael Stevens?

  Yeah.

• 45:29Hannah Fry?

  Is watching videos about primitive technology.

• 45:31Michael Stevens?

  Uh huh.

• 45:32Hannah Fry?

  Do you remember the guy who was in Australia, who would be there just in his shorts and like, every single tool he would, he had a knife and that was it. That was all he was allowed to have.

• 45:41Michael Stevens?

  Yeah.

• 45:42Hannah Fry?

  He would build the most unimaginably sophisticated, I mean practically entire villages, right? Pottery, you know, roof tiles, fires, I mean, weapons, all sorts of stuff. I think that the sort of collective knowledge of things that humans invented and created, it's very, very quick, I think, that it becomes this vast wealth of stuff that you would have to potentially learn.

• 46:08Michael Stevens?

  Yeah. Again, I think that you're gonna need to look at some like Stone Age time period to even be able to wrap your head around all of the skills, techniques and observations that humans collected and weren't just simply born with. Like knowing to drink when you're thirsty, you know, if that kind of stuff counts, then you can go back pretty far and we all had that knowledge. But, you know, I met the primitive technology guy.

• 46:36Hannah Fry?

  Did you?

• 46:37Michael Stevens?

  Yeah. When Adam and I were doing Brain Candy Live in Australia, he came to the show and that was, that was awesome.

• 46:45Hannah Fry?

  Did he have to announce himself or did you know immediately who he was? Because you don't really see his face that much on camera, do you?

• 46:50Michael Stevens?

  No, no, no, no. But he had been in contact with Adam Savage. And so that's how it like happened and Adam met him and I just kind of tagged along and was like, hello, me too, hello.

• 47:02Hannah Fry?

  Imagine Vsauce tagging along and being like me too. Imagine, rather than the central character.

• 47:08Michael Stevens?

  I was like, sorry to butt in, but I just wanted to shake your hand. Yeah, that's what it was. It was really cool.

• 47:15Hannah Fry?

  The other thing I've become quite obsessed with is like ancient chemistry.

• 47:20Michael Stevens?

  Yeah.

• 47:21Hannah Fry?

  My Saturday mornings, that's the treat that I give myself, a little bit of a lie in and watching videos about ancient chemistry techniques, ancient technology. I'll tell you what, we'll link some of my favorite channels in the description below.

• 47:34Michael Stevens?

  Now, could there come a point where the past is so big and new discovery is so slow that our species is actually forgetting things faster than we're learning new things? That'd be a good like science fiction future premise for a book.

• 47:52Hannah Fry?

  Yeah.

• 47:53Michael Stevens?

  Because there's certainly a lot of things people used to know that we literally no longer know.

• 47:57Hannah Fry?

  That we've outsourced basically to technology instead, really.

• 48:01Michael Stevens?

  Yeah, that we've outsourced or we literally have lost. Like, we don't know what they did. We don't know what they put in this concrete to make it so strong.

• 48:13Hannah Fry?

  Well, that was an absolutely delightful field notes. We went backwards in time, we went forwards in time, we went through wormholes and around the other side of the other side of the moon. I hope you enjoyed it too. Told you it wasn't going to be boring. Told you it was going to be, this is not going to be the one where we've got zero views on YouTube, Michael. We'll have to save that one for another time.

• 48:31Michael Stevens?

  That one is coming. We're going to crack that nut. We're going to figure it out. Everyone's trying to beat the algorithm and we are going to try to be beaten by the algorithm.

• 48:40Hannah Fry?

  Do you know that is that there is that is quite a nice game theory idea, isn't it? Like whether we could actually do it, whether we could get everybody to collectively agree to not watch one video.

• 48:49Michael Stevens?

  Yeah, how would you, but see, there's going to be the reverse psychology of it, the psychological reactance of it. Where if you tell people not to watch it, they're going to make them want to watch it more.

• 48:58Hannah Fry?

  Yeah, I know.

• 49:00Michael Stevens?

  Honestly, it would have to be like an advertisement that we desperately want everyone to watch. Like, this is an ad for a really boring product and we were paid to do this and please watch it and people are like, no. Literally no.

• 49:12Hannah Fry?

  We'll think on that. We'll ponder on that. If you have any suggestions in the comments of how we can get zero reviews, then let us know. I'd be well up for that. It'd be great. But anyway, anything else that you have, let us know in the comments or send them to us at the rest is science at gullhanger.com. We love hearing from you.

• 49:29Michael Stevens?

  And we'll see you next time.

• 49:31Hannah Fry?

  Bye bye.

• 49:44Speaker 03

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